Regularly spaced TiO2 nanotubes were prepared by anodizing a titanium substrate in triethylene glycol electrolyte at elevated temperature. In comparison to conventional TiO2 nanotubes, spaced nanotubes possess an adjustable spacing between the individual nanotubes; this allows for controlled buildup of a hierarchical nanoparticle-on-nanotube structure. Here, we use this principle for layer-by-layer decoration of the tubes with TiO2 nanoparticles. The hierarchical structure after N doping and NH3 treatment at 450 °C shows a significant enhancement of visible-light absorption, although it only carries a low doping concentration of nitrogen. For optimized N-doped and particle-decorated spaced TiO2 nanotubes, a considerable improvement in photocatalytic activity is obtained in comparison with conventional N-doped TiO2 nanotubes or comparable N-doped nanoparticle films. This is attributed to an enhanced visible-light absorption through the N-doped nanoparticle shell and a fast charge separation between the shell and the one-dimensional nanotubular core.
Keywords: hierarchical structures; nitrogen doping; photocatalysis; spaced TiO2 nanotubes; visible-light absorption.